At pH values above 7.5, the disinfectant capability of chlorine is
greatly reduced
Chlorine's disinfecting power in water depends on the relative proportion of two chemical species formed when chlorine dissolves in water: hypochlorous acid (HOCl) and the hypochlorite ion (OCl-).
HOCl is an uncharged, small molecule that can penetrate microbial cell walls efficiently, making it a far more powerful disinfectant, whereas the negatively charged OCl- ion is repelled by the negatively charged surface of bacterial cells and is therefore a much weaker germicide, often cited as 60-80 times less effective than HOCl.
The equilibrium between HOCl and OCl- is strongly pH-dependent: at lower pH (below about 6.5-7), HOCl dominates, while as pH rises, the equilibrium shifts progressively toward OCl-.
At pH values above 7.5, the dissociation strongly favours OCl-, so the proportion of the more effective HOCl drops sharply.
Consequently, the disinfectant capability of chlorine is greatly reduced at pH above 7.5, which is why water and wastewater treatment plants generally try to maintain slightly acidic to near-neutral pH for effective chlorination.
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